CH. LIX.] 



OOGENESIS 



877 



We are now in a position to construct a diagram similar to that 

 in fig. 560, which represents the history of the male reproductive 

 element. The history of the female element from the original germ 

 cell is shown in fig. 563. 



As in the male the germ cell divides and subdivides, and in this 

 way successive generations of oogonia are produced ; two of 

 the subdivisions are shown in. the diagram. Each oogonium then 

 grows, and becomes a primary oocyte, which is the ovarian ovum 



Germ cell 



Oogonia 



- - Period of growth 

 Primary oocyte 



-First polar body 



j Division of the 

 I First Polar body 



Second polar body 



Heterotype mitosis 

 and reduction of the 

 chromosomes to half 

 the usual number 



Secondary oocyte 



Mature ovum 



FIG. 563. Diagram to illustrate oogenesis. 



within the Graafian follicle. Its maturation, which occurs outside 

 the ovary, is represented in the lower part of the diagram, which is a 

 repetition of what is also seen in fig. 562. 



Each primary spermatocyte gives rise to four spermatozoa of 

 equal value. Each primary oocyte gives rise to four cells, which are 

 of unequal value ; one of these is the mature ovum, the other three 

 are the minute polar bodies which may be regarded as abortive ova. 

 The mitosis which results in the halving of the normal number of 

 chromosomes occurs in each sex at the same stage, namely, in the 

 formation of the secondary spermatocyte, and of the secondary 

 oocyte respectively. 



